Method of controlling molecular weight of polyacrylonitrile produced in aqueous salt solutions



METHOD F CONTRQLLlNG MULEQULAR WIEIIGHT 0F POLYACRYLQNITRILE PRO- DUCEDTN AQUEUUS SALT SQLUTIQNS Clyde W. Davis, Antioch, Caliil, assignor toThe Dow Chemical (Iompany, Midland, Mich a corporation of Delaware NoDrawing. Application August 25, 1952, Serial No. 306,310

4 Claims. (Cl. 260--S8.7)

This invention relates to a method of controlling the molecular weightof the polymer obtained when acrylonitrile is polymerized in an aqueoussaline medium which dissolves both the monomer and the polymer.

It is known that acrylonitrile may be polymerized in concentratedaqueous solutions of certain salts to form solutions of the polymer, andthat the polymer solutions may be used to spin fibers. The saltsolutions which may be used include the zinc halides, stannic halides,and the calcium or sodium thiocyanates, as disclosed in the art. Inaddition, some solutions of mixed salts can be used in which one salt iscapable at higher concentrations of dissolving the polymer and the othersalt is incapable alone of dissolving the polymer at any concentrationof that salt in water. Examples of such useful mixtures include zincchloride and lesser amounts of calcium chloride, zinc chloride andlesser amounts of magnesium chloride, zinc bromide and lesser amounts oflithium chloride, sodium thiocyanate and lesser amounts of potassiumthiocyanate, as described by the present inventor and others incopending application Serial No. 228,752, filed May 28, 1951, now U. S.Patent No. 2,648,647. The salt concentration in the useful solutions, asshown in said patent, must be at least 55 per cent but need not exceed65 per cent, by weight.

In carrying out the polymerization of acrylonitrile in aqueous saltsolutions, it is observed that the rate of polymerization varies in aseemingly erratic manner from batch to batch, and that the molecularweight of the polymer obtained is correspondingly inconsistent. Thisanomalous condition persists despite careful duplication of theconcentrations of the saline medium, monomer, and catalyst and of thereaction temperature. For a polymerization process to be commerciallyacceptable, it must give consistent results and conditions must be knownwhich will yield a product whose molecular weight is in any desiredrange. This is especially true in the case of polymerizaion processesintended to produce a fiber-forming polymer, as the fibercharacteristics depend to a large extent on the chain length of thepolymer employed and upon the viscosity of the spinning solution. Themethod used in the past for preventing the formation of extremely largemolecules has involved introduction of mercaptans or of unsaturatedcompounds which are copolymerizable with acrylonitrile and which serveas chain stoppers. Such organic agents necesarily alter the chemicalcharacteristics of the polymer as well as its molecular weight, and suchalteration is not always considered desirable. I It is accordingly theprincipal object of the present invention to provide a method wherebythe polymerization of acrylonitrile in aqueous saline media may beeffected so as to give consistently reproducible results. A relatedobject is to provide a method whereby the molecular Weight of thepolymer obtained may be controlled at will within reasonable limits.

According to the present invention, the foregoing and related objectsare attained through control or adjustment of the concentration ofcopper ions in the saline polymeriice 2 zation medium. Morespecifically, it is found that, to obtain polymers with molecularweights controlled in the range from 12,000 to 125,000, the salinemedium should contain correspondingly from 1.0 to 0.02 part of copperions per million partsby weight of the salt solution. Similarly, themost generally useful polymers, having molecular weights in the rangefrom about 20,000 to about 70,000 are obtained consistently when thereis present in the saline medium from about 0.5 to about 0.03 part ofcopper ions per million parts of the salt solution.

It has been found that, of the commonly occurring impurities in zincchloride, copper is the only one having any significant adverse effecton polymerization rate and molecular weight. This is illustrated in thefollowing table. In each test, a 60 percent solution of zinc chloride inWater was used. The solutions were adjusted to the neutral point forzinc chloride, to make them substantially free from zinc oxide and'fromexcess hydrogen chloride. The zinc chloride was fairly, pure, butcontained a few tenths of a part of copper per million parts by weightof the solution. The amount of copper present in the zinc chloridesolutions was too little to prevent polymerization but was sufiicient tokeep the molecular weight of the polymer near 30,000 when no otherimpurities were present. The test ion was added to the zinc chloridesolution in an amount representing 5 parts of that ion by weight permillion parts of the solution. One part by volume of acrylonitrilemonomer was dissolved in six volumes of the so-modified salt solution,and 1 per cent by weight of potassium persulfate, based on the weight ofmonomer, was used in each case as the catalyst. Polymerization waseffected at 50 C. After a fixed period, the viscosity of the resultingpolymer solution was measured, the percent polymer solids in thesolution was determined, and a sample of the polymer was dissolved toform a standard solution in dimethyl formamide and its molecular weightwas calculated from the specific viscosity of the standard solution,using the equation and constantgiven by Houtz in the Textile ResearchJournal, vol. 20, page TABLE I Viscosity Percent of Polymer PolymerMolecular contaminating ion solution at in zinc Weight of 50 0.,chloride Polymer Poises solution None of the added ions, except that ofcopper, adversely aitected the polymerization rate at the concentrationemplayed, as may be noted from the consistent production of solutions ofnearly the same concentration as that found in the blank. Each of thosesolutions was of a practical spinning viscosity, except when copper waspresent, and the polymer recovered from each of the solutions had amolecular weight of the same order of magnitude as that made in theoriginal zinc chloride, except that made in the solution containing 5parts per million of the cupric ion as an impurity.

in further series of determinations, it was learned that the molecularweight of the polymer and the viscosity of the saline solution of thepolymer could each be varied and controlled at will by variation andcontrol of the concentration of cupric ion in the saline medium usedduring polymerization. This is illustrated in the follow.- ing typicalexamples in each of which the salt solutions employed had beenespecially purified to free them initially from all detectable copper(below 0.01 part per million) and had been adjusted to the zinc chlorideneutral point. Controlled amounts of copper (as cupric chloride) wereadded to portions of the solutions, and the effects of the copper werenoted by determination of the molecular weights of the several polymersamples.

Example 1 Acrylonitrile was polymerized at 50 C. in each of several 60per cent solutions of zinc chloride in water and in the presence of 0.15per cent by weight of hydrogen peroxide based on the weight of monomer.The solutions contained the indicated concentrations of cupric ion.

Average Molec ular Weight of Polymer Parts per million of Cu in solutionabout 5, 200

Example 2 The procedure and conditions of Example 1 were followed exceptthat the polymerization medium was a solution in water of 45 per centzinc chloride and per cent calcium chloride.

Avera e Melee ular Veight Parts per million of Cu in solution of Polymer26, 500 about 17,000 about 12, 000

about 4, 200

Example 3 Acrylonitrile was polymerized at 50 C. in each of severalsolutions in water of 45 per cent zinc chloride and 15 per centmagnesium chloride, using 1 per cent of potassium persulfate ascatalyst, based on the weight of monomer. The eifect of variation ofcopper ion concentration was observed as before.

Average Molecular Weight Parts per million of Cu in solution of Polymerover 150,000 about 125, 000

It is to be understood that, owing to difierences in analyticalprocedures, both as to copper concentration and as to molecular weightdetermination, itshould not be expected that the precise values given inthe examples will be obtained each time the work is repeated. Theresults will be of the same order, however, and it is believed that thedesirability of limiting the copper con centration between 0.02 and 1part per million, and preferably between 0.03 and 0.5 part per million,will be readily apparent.

Similarly, when acrylonitrile was polymerized at various volume ratiosof monomer to salt solution, using the same mixture of zinc chloride andmagnesium chloride as in Example 3, the viscosity of the resultingsolutions varied as follows with the cupric ion content of the medium.

TABLE II Viscosity of polymer solution, poises, at 50 0.

Concentration Cu, parts Thus, by control of the copper ion concentrationin a salt solution used as the medium for polymerizing acrylonitrile, inwhich zinc chloride is the principal solute, it is possible to controlthe molecular weight of the polymer within practical limits and, whenusing such polymerization media, it is possible to obtain a polymersolution of any desired spinning viscosity through selection of theappropriate initial ratio of monomer to the salt solution.

The invention is not limited in its applicability to the particular saltsolutions identified in the examples but is of equal value when usingany salt or mixture of salts whose aqueous solution is a solvent forboth monomeric and polymeric acrylonitrile.

I claim:

1. The improvement, in the process of polymerizing acrylonitrile whilethe latter is dissolved in an aqueous inorganic salt solution which is asolvent for the monomer and for the polymer, and in which the saltconcentration is from 55 to 65 per cent by weight and the principalsaline solute is zinc chloride, which consists in carrying out theprocess in such a medium containing from 0.02 to 1.0 part by weight ofcopper ions for each million parts by weight of the salt solution.

2. The improvement claimed in claim 1, wherein the aqueous salt solutioncontains zinc chloride as the principal solute, together with a lesseramount of calcium chloride.

3. The improvement claimed in claim 1, wherein the aqueous salt solutioncontains zinc chloride as the principal solute, together with a lesseramount of magnesium chloride.

4. The improvement claimed in claim 1, wherein the characterising saltin the aqueous salt solution consists essentially of zinc chloride.

References Cited in the file of this patent UNITED STATES PATENTS2,356,767 Kropa Aug. 29, 1944 2,379,431 Fryling July 3, 1945 2,477,784Britton et al Aug. 2, 1949 2,486,943 Hammer et al Nov. 1, 1949 2,601,293Howard June 24, 1952 2,648,647 Stanton et al Aug. 11, 1953 FOREIGNPATENTS 599,472 Great Britain Mar. 12, 1948 OTHER REFERENCESBoundy-Boyer: Styrene, 285, 286 (Reinhold, 1950).

1. THE IMPROVEMENT, IN THE PROCESS OF POLYMERIZING ACRYLONITRILE WHILETHE LATTER IS DISSOLVED IN AN AQUEOUS INORGANIC SALT SOLUTION WHICH IS ASOLVENT FOR THE MONOMER AND FOR THE POLYMER, AND IN WHICH THE SALTCONCENTRATION IS FROM 55 TO 65 PER CENT BY WEIGHT AND THE PRINCIPALSALINE SOLUTE IS ZINC CHLORIDE, WHICH CONSISTS IN CARRYING OUT THEPROCESS IN SUCH A MEDIUM CONTAINING FROM 0.02 TO 1.0 PART BY WEIGHT OFCOOPER IONS FOR EACH MILLION PARTS BY WEIGHT OF THE SALT SOLUTION.